Most laser light sources emit polarized light during operation. While a polarized state of light can be advantageously used in some applications, in other applications it is detrimental. For example, for Raman amplification of an optical signal in a non-polarization maintaining optical fiber of a fiberoptic communication link, a depolarized Raman pump light source is needed. This is because a Raman amplification process is sensitive to mutual polarizations of the signal and the pump, which tend to be randomly fluctuating in non-polarization maintaining fibers.
A common approach to depolarizing a polarized light beam having a coherence length L is to split the beam into two orthogonally polarized sub-beams, delay one of the sub-beams by a length larger than L relative to the other sub-beam, and recombine the sub-beams into an output optical beam. This causes the correlation of phase between the sub-beams to be lost, which effectively scrambles the output polarization. A polarization beam combiner is sometimes used in combination with the depolarizer, to combine and depolarize optical beams of two laser diodes at the same time. Using two laser diodes instead of one allows one to increase the output power of the depolarized optical beam, and to improve reliability via redundancy.
Referring to FIGS. 1A and 1B, a prior-art depolarized laser light source 100 includes first and second laser diodes 71 and 72 coupled via polarization-maintaining optical fibers 73 and 74 to input ports 81 and 82, respectively, of a polarization beam combiner 8. A birefringent crystal 10 is coupled to an exit port 83 of the polarization beam combiner 8. In operation, lightwaves from each of the laser diodes 71 and 72 impinge onto the birefringent crystal 10, as shown in FIG. 1B. The crystal principal axis of the birefringent crystal 10 is disposed at 45 degrees with respect to the polarization directions of the combined lightwaves. Each of the combined lightwaves is split within the birefringent crystal 10 into two waves having orthogonal linear polarizations, one of which is delayed by a length L with respect to the other. When the delay L is larger than each coherence length of the lightwaves generated by the laser diodes 71 and 72, the combined lightwaves in an output fiber 75 are depolarized. The light source 100 has been disclosed by Matsushita et al. in US Patent Application Publication 2002/0141698.
One drawback of the light source 100 of Matsushita is that it usually requires a very long birefringent crystal 10. By way of example, a Raman pump laser diode manufactured by JDS Uniphase Corporation of Milpitas, Calif., USA, has a coherence length of 60 mm. When using YVO4 crystal 10 having Δn=ne−no≈0.2, one would require the YVO4 crystal 10 to be at least 60 mm/0.2=300 mm long to depolarize the light emitted by this Raman pump diode. Such a long crystal is impractical to grow.
Ziari et al. in U.S. Pat. No. 6,522,796 disclose a light source similar to the light source 100 shown in FIG. 1. The Ziari device uses a polarization beamsplitter cube in place of the polarization beam combiner 8, and a length of polarization-maintaining (PM) optical fiber in place of the birefringent crystal 10. A polarization maintaining fiber is also used by Fukushima in U.S. Pat. No. 5,692,082 to depolarize laser diode light. The length of the PM fiber must be large enough, so that the optical path difference (OPD) between the orthogonal polarization modes in the PM fiber is greater than the coherence length of the light source. By way of example, for the above mentioned Raman pump laser diode, a required length of a typical PM fiber with the birefringence Δn=ne−no≈3.7×10−4 should be at least 60 mm/3.7×10−4=160 m. A 160 m long PM fiber is lossy, expensive, and bulky.
Fidric et al. in U.S. Pat. No. 6,870,973 disclose a method allowing one to reduce the required length of the PM fiber. In a depolarized light source of Fidric et al., polarizations of multiple longitudinal modes of a Raman pump laser diode are overlapped, by converting half of the longitudinal modes to an orthogonal polarization state. As a result, a significantly shorter PM fiber length is required. The coherence length of this laser is only 9 mm, thus requiring only 24 m of PM fiber, or only 44 mm long YVO4 crystals. However, these length values are still too long for constructing a compact and inexpensive depolarized light source.
Another approach, taken by Yao et al. in US Patent Application Publication 2009/0225420, is to create the required optical path difference in a bulk-optic delay line or in a Michelson interferometer based on a polarization beamsplitter cube. The beams of orthogonal polarizations propagate along different directions in different optical paths, and one of the beams is delayed with respect to the other in a dedicated delay line. Optical path differences of tens of millimeters can easily be created in a bulk-optic delay line. Detrimentally, Michelson interferometers require complex optomechanical packaging to ensure stable operation.
Tselikov et al. in U.S. Pat. No. 6,574,015 disclose a depolarizer based on a pair of polarization beam splitters and a fiberoptic delay line. One of the two orthogonally polarized sub-beams propagates in free space, and the other is coupled to a length of optical fiber. However, a fiberoptic delay line can create an unwanted temperature dependent variation of optical loss in one of the two optical paths for polarized sub-beams.
Most of the above described depolarizers and beam combiners use optical polarizing beamsplitter cubes. In a polarizing beamsplitter cube, the orthogonally polarized incoming and/or outgoing optical beams are disposed at 90 degrees to each other. Since the inputs and outputs of the beam combiners and depolarizers are usually coupled to an optical fiber, the overall size of the device is increased due to a requirement to route all optical fibers on one end of the package, while observing a minimum bending radius of an optical fiber.
Walk-off crystals can be used for combining or splitting orthogonally polarized beams. For example, Ziari et al. in U.S. Pat. No. 6,522,796 disclose, as an alternative, a polarization beam combiner having parallel input optical fibers coupled to a walk-off crystal through a couple of adjacently disposed lenses, thus not requiring the optical fibers to be bent within the package. This polarization beam combiner must use a walk-off crystal of sufficient length to create enough lateral displacement to accommodate two adjacent collimating lenses for coupling light into parallel fibers. For example, ˜20 m YVO4 crystal would be required to combine two orthogonal polarized beams spaced 2.0 mm apart. It is desirable to further reduce size of a polarization beam combiner.
Therefore, the prior art is lacking a compact, stable, reliable, and inexpensive depolarizer, especially a polarization beam combining depolarizer.